Formation history of the Japanese Islands
Late Miocene-Pliocene
Fig.
Pliocene
paleogeographic map [
]
The volcanic front and the trenches were situated near the present
positions and the shoreline was similar to the modern line in this
period. The Japanese Islands, except for a few areas including
Beppu-Shimabara Graben, have been dominated by compression owing to the
subduction of the Pacific Plate and the Philippine Sea Plate since this
period. As a result, reverse or strike-slip faulting and folding formed
mountain ridges and basins. The active island-arc volcanism occurred;
in northeastern Honshu and Hokkaido, eruptions with large scale
pyroclastic flow produced volcanoes with a caldera, and in Kyushu,
grabens developed in the Hohi volcanic zone (on the north of the
Beppu-Shimabara Graben), which may be regarded as the pre-subsidence of
the Okinawa Trough extending to the southwest of these grabens.
In northeastern Japan, a zone along the
Ou Range started to uplift 10
million years ago and then emerged six million years ago. Since the
uplift rate increased in the Late Pliocene, depression areas in the
Miocene became high elevation areas inversely at present. The mountain
areas (Dewa Mountains) on the west of the Ou Range upheaved above
the sea level 2.5 million years ago (Late Pliocene).
In southwestern Japan, tectonic movement was milder compared to
northeastern Japan. Gentle warping in the island arc direction occurred
in the inland five million years ago. Lacustrine and fluvial deposits
accumulated in depression areas formed by this warping. The developed
depression area resulted in the
Seto Inland Sea. Hardly any Neogene
deposits except shallow-marine sediment of the Middle Miocene are found
in northern and central Kyushu. Therefore, Kyushu is thought to have
been broadly land. In the Nansei Islands, the Okinawa Trough began to
be formed in the Late Miocene-Pliocene (or 10 million years ago).
In central Honshu, the collision of the Izu-Bonin Arc, which started 15
million years ago, still continued. The Tanzawa and Izu blocks accreted
to the Honshu island to be Tanzawa Mountains eight million years ago
and Izu Peninsula 1.5 million years ago, respectively. These multiple
collision of the blocks markedly deformed the Honshu island, bended
geotectonic structure such as the Shimanto Belt, the Chichibu Belt, and
the Median Tectonic Line. The collision also effected on major
configuration including the Akaishi, Kiso, and Hida Mountain Ranges to
the west of the collision zone.
Quaternary
Fig.
Pleistocene paleogeographic map []
The Japanese Islands has been strongly compressed by the eastward
movement of the sea floor of the Sea of Japan as well as the westward
subduction of the Pacific and Philippine Plates since two million years
ago. The uplift rates of mountain ranges formed between the end of the
Miocene and the Pliocene increased because of this powerful compression. As a
result, an abundance of clastic material from mountains was provided in
lowlands. There are also considerably subsiding areas. The
Kanto Plain,
for example, has sunk over 1000 m in the Quaternary, being the largest
plain in Japan. The uplift rates of mountains in northern and central
Honshu (Tohoku and Chubu Regions) are greater than those in western
Honshu (Chugoku Region).
The world repeatedly experienced ice ages in the Quaternary. The
Japanese Islands were connected with the continent by land during the
last ice age (tens of thousands to 10000 years ago) because the sea
level fell by 120 m. Mammoths came to Hokkaido from Siberia, and various
animals came and went between southwestern Japan and the Korean
Peninsula. Glaciers developed in high mountain areas of Hokkaido to
central Honshu, forming glacial landforms. Some of them remain in those
areas.
Most Quaternary volcanoes are active in current positions and create
characteristic landforms of the Japanese volcanic islands.
